a) Field of the Invention
The invention is directed to a method and a device for detecting fault current arcing in electric circuits for consumers with relatively constant power consumption, in particular for connected heating elements. The invention is applied for early detection of arcing resulting from insulation defects occurring in any voltage power supply at any power supply frequency, but particularly in onboard aircraft power supplies, due to high mechanical, chemical, electrical and/or thermal loading.
b) Description of the Related Art
Fault current arcing, or ‘arc tracking’, as it is called in English-language technical literature, is particularly dangerous above all in aircraft wiring for safety reasons.
The typical pattern is that insulation faults occur in the wiring of electrical consumers, particularly heating devices, due to pre-existing defects (penetration, e.g., by mechanical, chemical and/or thermal alternating loads), initiate arcing fire, and often destroy an entire area of the cable or consumer. Often, shunts (leakage currents) with an initially high impedance occur and—usually aided by electrolytically acting liquids—lead to wet arc tracking, as it is called, subsequently decrease in impedance over the course of time and, finally, result in high-energy short-circuit arcing. A set of problems arises in that the environment is severely compromised by a cable fire initiated in this way and explosive sequential faults can cause catastrophic air accidents.
Alternately, so-called dry arc tracking can also occur and (e.g., due to chafing of cable on sharp edges) can cause sudden low-impedance shunts (short circuits) that can lead to the same catastrophic results.
The chief problem in developing safeguards for controlling a very high-current load (e.g., an in-line heater) is that the initial currents of fault current arcs (hereinafter, for the sake of brevity, arcing) can be very small compared to the rated currents of the load in use. Tests in which the fault current of an electric arc initially has a value of Ieff<50 mA at a rated current of 3.5 A have shown that the fault current attained considerably higher current values within a short time during the course of the experiment. On the other hand, for purposes of early detection of fault arcing, it must also be considered that the availability of aviation equipment and aviation systems is very important and, therefore, false triggering of safety measures cannot be allowed to occur.
There are diverse publications that particularly address this subject in aviation and aerospace. However, no satisfactory solutions have yet been found. But there has been a concerted effort to develop a safeguard which detects the occurrence of arcing before the conventional overcurrent protection (which sometimes does not interrupt the current until after a cable fire has already started) and which turns off the defective load circuit already before the actual short circuiting and before the occurrence of greater damage.
For example, WO 01/90767 A1 describes a method and a device for detecting arc faults (arc tracking) in which an AC signal in a cable is sampled in discrete time and converted into a trigonometric function of the alternating current based on the discrete current signal by interpolating a determined quantity of sample values. The presence of leakage current arcing is determined by comparing the actual AC frequency determined in this way to a reference frequency. Interference caused by turn-on processes of other consumers is distinguished in that its decay behavior exhibits a different frequency pattern.
This solution is disadvantageous in that the signal interpolation of the frequency response—in order to be fast—can rely on only a few current measurement values. Also, the similarities to interference in frequent turn-on processes at the supply voltage still pose a relatively high risk of erroneous triggering of the safeguard. Further, the signal processing is relatively elaborate, easily subject to error, and can only reasonably be applied at all for AC power supplies with a known, constant power supply frequency.
DE 199 53 354 A1 discloses a device for detecting leakage current arcing, particularly in household circuits, which has an application-specific circuit (ASIC) based on current measurement and correlation of the actual current signal with a noise-free AC signal component in order to determine leakage current arcing from the correlation deviation and interrupt the circuit. The filtering for removing the noise component is carried out by digital signal processing using a standard processor unit (CPU). It is at least doubtful that interference caused by turn-on processes of other consumers can be safely distinguished from interference caused by arcing, so that erroneous triggering of the protective circuit cannot be ruled out.
A somewhat different approach is taken in the solution according to DE 698 13 842 T2, wherein the current in cable bundles of a household distribution system is monitored by means of a transformer and the rate of change of the current dI/dt is observed. A test signal representing a leakage current arc signal simulated by a signal generator is fed over the same transformer. Following a comparison of the signal changes, the number of times that a threshold value is exceeded is counted and, after a determined quantity, is used to trigger an interrupter signal.
In US 2002/0149891 A1, a detection signal for arcing is received, likewise over a transformer, and is analyzed along different filter paths. The continuous waveform is taken off as voltage from the secondary winding of the transformer and branched into two filter paths. In the first filter branch, the AC frequency component and, in the second filter branch, the high-frequency component of the transformer output are separated from the input signal, wherein the high-frequency component is used to determine the presence of leakage current arcing. For this purpose, the signals of the two filter paths are rectified after a two-stage filtering and are divided in peak signal values and average signal values, wherein particularly high peak values in one of the two filter branches leads to an immediate triggering of the protection circuit.
This method is disadvantageous in that it is only suitable for AC power supply and, in addition, presupposes a relatively low power supply frequency. The analog processing in the filter paths is complicated and relatively prone to interference signals so that erroneous triggering of the protection circuit cannot be ruled out with certainty.
A solution that was conceived specifically for aircraft and also for the 400-Hz power supply specific to aircraft is disclosed in DE 100 17 238 A1 (analogous to U.S. Pat. No. 6,625,550 B1; U.S. Pat. No. 6,782,329 B1). In this case, the protection circuit is triggered after a specific “current signature” of the usually very specific (almost standardized) consumers in an aircraft, wherein a rate of change of a circuit-characteristic signal is observed as input signal and, after signal processing steps in an ASIC by means of a microprocessor, a selection of signal characteristics typical of the consumer is filtered out so that only “current signatures” of the monitored circuit associated with those of leakage arcing lead to immediate triggering of the protection circuit.
This signal processing is disadvantageous in that the programming of consumer-specific “current signatures” requires the use of a processor and the large proportion of analog processing steps (integrator, bandpass filter, voltage zero crossing detection, determination of the proportion of high frequencies) increases the susceptibility of the detection circuit to interference.
All of the above solutions have in common that they evaluate substantially only a current-equivalent signal quantity of the monitored load circuit. High-impedance leakage current arcing is difficult to detect in this way and unnecessary erroneous triggering of the safeguard, which poses a danger vis-a-vis the safety requirements in aircraft, cannot be reliably excluded. Moreover, most of the known solutions comprise predominantly analog evaluating circuits, which are more prone to problems.